1. Field of the Invention
The present invention relates to a variable magnification lens useful for plate-making, copy-making etc. which is able to change the magnification of image in a range including one-to-one magnification.
2. Description of the Prior Art
In the past there are known such variable focus lenses useful for plate-making which is variable in magnification at a definite object-image distance. Some typical examples thereof are disclosed in Japanese Laid Open Patent Application No. 49,453/1973 (the counterpart of which is U.S. Pat. No. 4,037,937) and Japanese Laid Open Patent Application No. 1,242/1974 (the counterpart of which is U.S. Pat. No. 3,973,831). These known lenses are all unsatisfactory in respect of covering angle (field angle) which is only 30 degrees. Because of the narrow field angle, the distance between object plane and image plane allowable for a certain size of plate to be made is limited and directly determined by the angle. It is impossible to select a smaller object-image distance than that. For this reason, it has been difficult to realize a further reduction of the size of plate-making apparatus. The size reduction of plate-making apparatus may be achieved by widening the field angle of the photographing lens. However, it brings forth another problem. The correction of aberrations becomes more difficult with broadening the field angle and, therefore, it is hardly possible to maintain good image forming performance of the lens system.